Question

Which of the following is not a colligative property?

• A. Osmotic pressure
• B. Depression of freezing point
• C. Elevation of boiling point
• D. Refractive index

Hint:

Colligative properties are independent of solute type and depend on solute particle concentration.

Answer 1

The Correct Option is D

Step 1: Define colligative properties
Colligative properties are:
- Osmotic pressure: The pressure needed to stop osmosis; it's directly related to solute concentration.
- Freezing point depression: The reduction in freezing point; it's directly related to molality.
- Boiling point elevation: The increase in boiling point; it's directly related to molality.
- Vapor pressure lowering: The reduction in vapor pressure caused by solute particles.

Step 2: Analyze each option
- Osmotic pressure: Dependent on solute particle count (van’t Hoff factor multiplied by concentration). This is a colligative property.
- Freezing point depression: Dependent on molality and the quantity of solute particles. This is a colligative property.
- Boiling point elevation: Dependent on molality and the concentration of solute particles. This is a colligative property.
- Refractive index: This measures light bending in a substance. It is influenced by the chemical composition of both the solute and the solvent, not solely the particle count. Therefore, it is not a colligative property.

Step 3: Conclusion
The refractive index is not classified as a colligative property.